The invention relates to a scanner device of the type for measuring the color properties of a measured object pixel by pixel by means of a color measuring head, which scanner device is able to move the color measuring head in at least one dimension across a measurement object to be measured, and the color measuring head has at least one illuminating channel and a collection channel, which illuminating channel has a light source and optical means for illuminating the measured object at a measurement site at a mean angle of incidence of 45°, and which collection channel has optical means for capturing measurement light emitted from the measured object at the measurement site at a mean collection angle of 0° and coupling it into a light guide which directs the captured measurement light to a wavelength-selective photoelectric transformer, preferably provided in the form of a spectrometer which resolves it into a number of wavelength ranges and generates an electric measurement signal corresponding to each wavelength range.
Published international patent application WO 2006/045621 A1 provides a detailed description of an automated measuring system for quality control and for controlling the color of a printing machine. It essentially comprises a measuring table on which a printed sheet to be measured can be placed, and a measuring unit comprising a beam-shaped line scanner and an individual color measuring head. The line scanner and individual measuring head can be moved above the measuring table by a computer-controlled drive mechanism so that it travels across every pixel of the printed sheet to be measured. The quality and control parameters needed for the printing process are derived from the resultant measurement values. The output parameters generally include color measurement values CIE XYZ and derived colorimetric variables (in accordance with CIE publication 15 and ISO 13655) as well as density measurement values in accordance with ISO 5. The individual measuring head is advantageously provided in the form of a spectral measuring head so that all the output parameters can be computed in a known manner from the measured spectral values of the reflection factor. As a rule, the individual measuring head is used to measure print control strips contained in the printed sheet, whilst the line scanner is used to scan the rest of the printed image content.
In practice, a measuring system of this type, especially the individual color measuring head, is required to satisfy very exacting demands.
In order to control printing machines during continuous printing, the measuring system must be capable of detecting the print control strips and sending feedback to the printing machine as rapidly as possible. This requires a very powerful optical measuring system to enable travel across the measured sample (the printed sheet to be measured) at high scanning speeds. The measuring system must also be immediately ready for use without a long warm-up time and transient response.
The size of the measurement field in the print control strip is tending to become increasingly small, on the one hand in order to save on waste and on the other hand to enable more measurement fields to be provided in the print control strip and control more printing systems with one strip.
Measuring small measurement fields cleanly at a high scanning speed requires a more complex design of measuring optics than is available in the prior art. Parameters such as the sensitivity to distance and positioning accuracy of the measuring head must satisfy significantly higher requirements. Allowance must also be made for the homogeneity of the measurement fields if working with small measuring orifices.
Requirements placed on the measurement range have risen with the introduction of highly pigmented inks, UV-dried inks, the use of high gloss materials and finishing processes involving lacquer coatings. This firstly requires a highly sensitive, low-noise measuring system. In addition, cross-talk from the surrounding area must be actively suppressed to enable effective use to be made of the large measuring range.
Toner powder is used within the printing range. There is no way of ruling out the possibility of the measuring optics becoming dirty over the course of time. Dirt exacerbates the problems associated with the high density measuring range and cross-talk.
The printed sheets are measured in the wet state, directly off the printing machine. The subsequent drying process changes the surface structure of the color layer and hence the measurement values. More stable measurement values can be obtained in a known manner by using orthogonal polarization filters in the illumination and measuring channels. These measuring techniques are used as standard for taking density measurements. Fast-scanning spectral color measuring systems incorporating polarization filter technology are not yet available on the market.
In order to achieve fast measuring cycles, system designs are desirable which are able to detect measurement values with and without polarization filters in one measurement pass.
Typical substrates (papers) used in the printing industry contain optical brighteners, which are excited in the spectral UV range and emit fluorescent light in the visible range. The fluorescent element of the substrate influences the color and density measurement values. Allowance needs to be made for these effects in the application and a stable process measuring technique is necessary. It is also desirable to characterize the proportion of optical brighteners in the paper more accurately to permit an alignment of the measurement values between different measuring devices with different lighting spectra.
In order to obtain a better characterization of a specific printing process, it is of interest to determine other parameters in addition to the reflection factor spectra. In the case of repeat print jobs, these might enable control of the reproduction quality to be improved or may be used to measure special materials (metallic reflecting substrate or metallic inks). Additional parameters include the rotation dependency of the substrate, the amount of optical brighteners in the substrate, measurement of the degree of translucence of the measured sample and a spectral or narrow-band gloss measurement.
The measuring system is used intensively in a production environment and should therefore contain as few parts as possible which might be susceptible to wear. All components, including the light source, must be designed for a maximum service life.
Since it is necessary to measure freshly printed print samples, it is important to have a geometry which hovers freely. The design of the measuring system must be such that it can be adapted to different paper thickness and take account of fluctuations in the planarity of the sample bed.
The scanning measuring systems known to date do not satisfy the requirements outlined above or satisfy only some of them or do so only to an unsatisfactory degree.
The AxisControl measuring system by the Heidelberg company uses a compact spectral measuring head with a two-dimensional drive across a planar sample sheet surface. The measuring head can not hover freely during measuring. The distance is controlled by mechanically placing the measuring head on the sample. The position of the control strip on the sheet must be manually determined with a light pen. The measuring technology is designed for spectral measurements without polarization filters only and typically requires a measuring field size of 5 mm×6 mm.
The Intellitrax measuring system by X-Rite Inc. combines a spectral color measuring head with a tracking sensor, which enables automatic tracking and positioning relative to the print control strip. The optical measuring system is designed so that it operates at a fixed distance from the test sample and must therefore tolerate the entire range of different paper thicknesses. When using standard 45°/0° geometry, the distance tolerances require there to be a correspondingly high peripheral distance between the illuminated and captured surface in the measurement field. For a broad range of paper thicknesses from 0 to in excess of 1 mm, this system restricts the size of the minimum measuring speckle and hence its use for detecting the smallest measurement fields without crosstalk.
The spectral measurement data is detected using a rotating filter wheel. A complete spectrum requires a measurement conducted over a time sequence with every filter position and corresponds to a complete rotation of the filter wheel. Taking measurements based on a time sequence involving a lot of spectral channels limits the minimum measuring time which can be achieved, which restricts the maximum scanning speed and the minimum measurement field size. During the scanning process, measurements are taken with the different filter channels at locally different positions. This positional offset may lead to fluctuations in the measurement values in the case of grid-patterned fields and a small measuring speckle. The measuring device enables a parallel measurement to be taken with and without polarization filters in one scanning pass. To this end, the sample is illuminated with polarized light and the filter wheel contains filters with and without polarization filtering. Since the measurement time is linked to the number of filters, only 3 density filters are used for the polarization filter measurements. The missing spectral values are numerically approximated on the basis of these 3 measurement values.
Another measuring system is described in document DE 195 30 185 C2. The measuring head contains lighting disposed at 0° with permanently fitted polarization filters and two different collection channels, namely a spectral measurement channel without polarization filters and a density measuring channel with polarization filters which can be read out in parallel. On the one hand, the fact of providing two spectral analysis systems in the collection channel leads to additional costs. On the other hand, a complete spectral measurement is not available using polarization filters. The two collection channels scan the measurement field at 45° from different spatial directions. Samples with measurement properties that are dependent on rotation are not evaluated equally by the two collection channels. The problem of distance is solved by creating an air cushion between the measuring head bottom edge and the paper surface, on which the measuring head hovers. A constant working distance is obtained by the air cushion irrespective of the paper thickness.